Galatians 3:15-18

Brothers, let me take an example from everyday life. Just as no one can set aside or add to a human covenant that has been duly established, so it is in this case. The promises were spoken to Abraham and to his seed. The Scripture does not say "and to seeds," meaning many people, but "and to your seed," meaning one person, who is Christ. What I mean is this: The law, introduced 430 years later, does not set aside the covenant previously established by God and thus do away with the promise.

(v15-17, NIV)

The passages referenced are Gen. 12:7; 13:15; 24:7. This may have been the next point brought up by the Jewish Christians. Since Paul is making a case for faith, they may object questioning whether the promise of God to Abraham still holds. God made a promise, and they believed it was their assurance of salvation and justification. They believed it put them above the Gentiles. Now, in Romans Paul talks about advantages that Jews have over Greeks, but in this respect we realize that all are saved by grace through faith. This new covenant allows Gentiles into the saving grace given by God.

Paul tries to use a human example to show them that this is not a contradiction. The human example is how the covenants that even humans make are not meant to be simply set aside or added to. In the same way, God's new covenant does not alter or set aside his old one. This is because the old covenant was fulfilled through Jesus. Jesus is the offspring mentioned in the initial promise. It is through Jesus that this covenant is fulfilled.

For if the inheritance depends on the law, then it no longer depends on a promise; but God in his grace gave it to Abraham through a promise.

(v18, NIV)

The Jews initially thought it was a promise to them. This is why Paul brought up the point that the word "offspring" is singular and not plural. However, another reason why it does not make sense is because it isn't a promise according to their way of thinking. If this is a promise to all of Abraham's offspring, then it requires that they all receive the promise otherwise it is no longer a promise; it becomes a broken promise. They also thought that it was through the law that they receive this promise. However, if the promise is directed towards all of them and they had to follow the law in order to receive it, the promise is again a broken promise. Nobody can fulfill the law in its entirety. Those who attempt to fulfill the law fail. Everyone has sinned. Therefore, they fail follow the law and nobody can receive the promise.